This project is concerned with the neuroscience of aging, and in particular with the response of the aging human circadian timekeeping system (CTS) to an abrupt shift in routine. The project will assess the adjustment of sleep, circadian rhythms, mood and performance in healthy elderly (55-90y) people to an acute (6h) phase delay in routine. This will allow the test of hypotheses regarding how well older people can cope with delays in routine that are often required either to remedy advanced sleep phase syndrome (ASPS) in which seniors have pathologically early bedtimes and waketimes, or to cope with evening- and night-working schedules which require sleep periods to be delayed by several hours. Knowledge of the aging circadian system's ability to phase delay and its effects on sleep, alertness, mood and performance is also of general interest. In all seniors, these entrainment processes are at work, ensuring that the CTS retains an appropriate temporal orientation, and a period length of exactly 24h, thus avoiding the episodic disruptions in sleep and daytime functioning that occur when the GTS runs at non-24h periods (as occurs, for example, in the profoundly blind). Fifteen-day experiments will be conducted in a controlled temporal environment in which the only time cues come from the experimenter. Thirty seniors, 15 younger seniors (55-70y) and 15 older seniors (71-90y) will each experience a surreptitious 6h phase delay in routine accomplished by "stretching" the waking part of day #6 by 6h, and holding each event thereafter (bedtimes, waketimes, meals, etc.) to a 6h phase delayed position (e.g., bedtimes at 0400 rather than 2200). Circadian rectal temperature rhythms (sampled every minute around the clock) will be used to track the adjustment of the CTS to the phase shift. All sleeps will polygraphically recorded, and mood, subjective activation, and performance assessed seven times per day. The phase delay condition studied in this project will be compared with a phase advance condition studied in 25 older seniors (71 -91 y) in an earlier experiment by the P.I. funded by the National Institute on Aging. The following specific hypotheses will be tested: 1) The GTS of seniors will adjust more slowly to phase delays than to phase advances; 2) this directional asymmetry will be more marked in older seniors than in younger seniors; 3) the phase shift will result in more disruptions in sleep and in daytime mood, alertness and performance in older seniors than in younger seniors.